The scalar projection of $\mathbf{a}$ onto $\mathbf{b}$ is $\mathbf{a}\cdot\hat{\mathbf{b}} = \mathbf{a}\cdot\dfrac{\mathbf{b}}{|\mathbf{b}|}$. This gives the component of $\mathbf{a}$ in the direction of $\mathbf{b}$, equal to $|\mathbf{a}|\cos\theta$ where $\theta$ is the angle between the vectors.